This is the very first anime produced by Tokyo Movie Shinsha. Based on a manga by Osamu Tezuka, directed by Osamu Dezaki, with music by Isao Tomita.
Like the 1960s episodes of Doctor Who, there are a lot of Big X episodes that are currently missing. For a long time the first 39 episodes were thought to be lost forever, and the Japanese VHS and laserdisc releases of the 1990s only included episodes 40-59. But just as missing Doctor Who episodes continue to be found, a 16mm print of Big X episode 1 turned up (oddly enough, in the hands of an American film collector), and more recently episode 11 was found as well. And now, you can finally enjoy the Big X origin story complete with English subtitles.
In the original manga Akira injects himself with the Big X formula in order to grow all big and super-heroic, but in the TV series the needle-pen injector has been changed into to a beam device, presumably to avoid an epidemic of copycat intravenous drug use on Japanese playgrounds. On the other hand, the first episode does feature a WWII flashback where Hitler offs himself with a bullet to the head.